Achieving a large contrast, low driving voltage, and high stability electrochromic device with a viologen chromophore†
The viologen radical salt (VRS), consisting of phenyl viologen (PV) dications and 2,2,6,6-tetramethyl-1-piperidinyloxy derivative (TEMPOD) anions, was successfully synthesized. The VRS was combined with N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) to form an electrochromic device (ECD). This ECD offers significant transmittance changes (>60% at both 580 and 620 nm), but only requires 0.4 V for switching, the lowest driving voltage ever reported. Such a low driving voltage further strengthens the energy-saving capability of the ECD. The incorporation of TEMPOD in the ECD greatly improved its write–erase ability while insufficient bleaching was clearly observed within only 5 cycles in the case of its counterpart without adding TEMPOD anions. No significant decay in the transmittance change was noticed in the proposed ECD after subjecting to 100 cycles. Even better cycling stability would be expected if counter anions with multiple TEMPO units were utilized.